This invention relates to a method of fabricating laminated elastic webs that are useful as elasticized structures, and more specifically, useful in outdoor applications such as window flashing.
Many disposable or non-disposable articles have laminated elastic components forming one or more expandable or stretchable portions in the article. For example, some of these types of articles include sweat bands, bandages, and laminated elastic waistbands in disposable diapers. Typically, the laminated elastic component of a disposable diaper is comprised of two layers of nonwoven fabric having elastomeric strands adhered therebetween. The elastomeric strands are laminated to the nonwoven layers in a pre-stretched condition. When the elastomeric strands relax, the nonwoven material gathers. The machines and tooling required for integral fabrication of laminated elastic articles are extremely complex.
Typically, the elastomeric strands and substrates are joined together by adhesives, such as hot melt pressure sensitive adhesives. Hot melt adhesives typically exist as a solid mass at ambient temperature and can be converted to flowable liquid state by the application of heat. In these applications, the hot melt adhesive is heated to the molten state and then applied to a substrate. A second substrate is then immediately laminated to the first and the adhesive solidifies on cooling to form a strong bond. The major advantage of hot melt adhesives is the lack of a liquid carrier, as would be the case for water-based or solvent based adhesives, thereby eliminating the costly drying step during application. Also, hot melt adhesives can be formulated to have relatively short open times, and thus do not require any curing and/or crosslinking. Thus, hot melt adhesives typically have high xe2x80x9cgreenxe2x80x9d strength upon application. Suitable hot melt adhesives must possess the appropriate bond strength to adhere the substrates involved, and must also possess adequate flexibility, staining or bleedthrough resistance, suitable viscosity and open time to function on commercial equipment, acceptable stability under storage conditions, and acceptable thermal stability under normal application temperature.
Many different polymers have been used in hot melt adhesives employed in the construction of laminates. In this regard, typical hot melt adhesives have employed polymers which have included styrene-isoprene-styrene (SIS); styrene-butadiene-styrene (SBS); styrene-ethylene-butylene-styrene (SEBS); ethylene-vinyl acetate (EVA); and amorphous poly-alpha-olefm (APAO). While these polymers, when properly blended, provide acceptable adhesion between most substrates employed in typical disposable goods construction such as diapers or packaging materials, they have several shortcomings which have detracted from their usefulness m connection with outdoor applications such as window flashing.
One of the most noteworthy shortcomings of hot melt adhesives concerns their durability. Typical hot melt adhesives do not perform well under conditions involving large temperature extremes such as outdoor applications where summer and winter temperatures can vary dramatically. Also, the long term aging, i.e. UV stability, of hot melt adhesives is also a concern with outdoor applications which are exposed to sunlight. Thus, it would seem logical to use an adhesive that provides long term strength, is UV stable and can perform well under wide temperature variances to bond a laminate structure together for use in outdoor applications. However, in order to obtain such characteristics, one must look toward curable or crosslinkable adhesives such as polyurethane based adhesives. Unfortunately, due to the need for curing and/or crosslinking, and thus the, time involved for curing and/or crosslinking, such adhesives have low xe2x80x9cgreenxe2x80x9d strength and would thus have inadequate bonding capabilities upon initial application. As a result, the use of curable or crosslinkable adhesives such as polyurethane in elasticized laminated webs is not practical since the web would fall apart after fabrication.
The present invention provides a method of making a laminated, gathered, elastic web. The method includes the steps of feeding a first substrate in a machine direction, feeding a second substrate aligned with the first substrate in the machine direction, and feeding an array of elastomeric strand material. between the first and second substrates in such a manner that the elastomeric strand material is stretched in the machine direction and aligned with the first and second substrates. A hot melt pressure sensitive adhesive, such as a styrene-isoprene-styrene based adhesive, is applied onto the elastomeric strand material, and a curable adhesive, such as a polyurethane based adhesive, is applied to one of the substrates. Thereafter, the two substrates and the elastomeric strand material are compressed to form a laminate elastic web while maintaining the elastomeric strand material in its stretched state. Machine direction tension is maintained on the laminate until the hot melt adhesive cools and bonds the layers together. Thereafter, the machine direction tension is released to permit the elastic web to contract to form a gathered elastic web.
The pressure sensitive hot melt adhesive is a thermoplastic adhesive that provides the green strength necessary to initially bond the laminated elastic web together while the curable adhesive provides long term strength for the structure over a range of temperature extremes, as well as excellent ultraviolet light stability which is desirable for outdoor applications such as window flashing. The pressure sensitive hot melt adhesive is a hot melt adhesive having sufficient strength to initially bond the elastic strands in place. One preferred example would be a hot melt adhesive used in bonding elastic strands in disposable articles, such as diapers. The curable adhesive may be any one of a variety of single component or dual component adhesives. The curable adhesive is preferably applied using hot melt application equipment. For example, if a single component system, the adhesive may be heat curable or moisture curable, but is preferably moisture curable polyurethane based. If a two component system, the curable adhesive may also be urethane based or may be epoxy based.
The substrates are preferably comprised of a spun-bonded high density polyethylene web and a low density polyethylene film. The pressure sensitive hot melt adhesive is applied at an add-on level of from about 2 to about 20 grams per square meter, but preferably about 15 grams per square meter. Likewise, the curable adhesive is applied at an add-on level of about 2 to about 20 grams per square meter, but preferably at a level of about 6 grams per square meter.
The method of the present invention thus overcomes the disadvantages of each individual adhesive, and provides a method for making a laminated, gathered, elastic web specifically adapted for outdoor applications. Various other features, objects and advantages of the invention will be apparent to those skilled in the art upon reviewing the following drawings and description thereof.